The Conservative leader - who unveils plans for the party's new online soapbox for voters today - faced embarrassment after it emerged that Tony Lit, the Tory candidate in Thursday's Ealing Southall by-election, had attended a Labour fund-raising event last month.

Mr Lit, 34, a flamboyant Sikh businessman, and his wife Mandy even posed for a photograph with Tony Blair. At the time, Mr Lit was the managing director of Sunrise Radio, which paid £4,800 for a table at the exclusive function for Asian businessmen.

The disclosure came as two new polls underlined how Labour was benefiting from the "Brown bounce", with the party opening up a seven-point lead over the Conservatives - its biggest in two years.

Mr Cameron insisted last night that he would not be distracted by short-term shifts in public opinion.

Tory officials will be worried to see Labour hit the vital 40 per cent mark in yesterday's ICM poll for The Sunday Telegraph. The poll put the Tories on 33 per cent and the Liberal Democrats on 19.

In a separate ICM poll for the News of the World, Labour was on 35 per cent, with the Tories trailing on 28 per cent. Asked which party was best equipped to run the country, 53 per cent said Labour under Mr Brown, while only 27 per cent opted for Mr Cameron's Tories.

The Tories' high hopes for Thursday's by-election in Ealing Southall suffered a knock following the disclosure of Mr Lit's attendance at the Labour fund-raiser.

Mr Lit, who joined the Conservative party only a few days before being unveiled as its candidate, tried to brush aside the controversy yesterday.

"As a businessman I did indeed attend this event for the Asian business community. But like many British Asians I feel the Labour Government does not have the answers to the challenges that currently face the country," he said.

Tory officials also played down the story and said the real story of the by-election had been how Mr Lit's energetic campaign had persuaded five Labour councillors to defect to the Conservatives.

Joan Ryan, the Labour Party vice-chairman who is running the by-election campaign, said: "David Cameron's personal intervention to overrule local Tories and appoint a candidate has totally backfired."

Lord Rennard, the Liberal Democrat chief executive, said the disclosure about Mr Lit was a big blow to the Tory campaign. "One has to wonder just what questions were asked before the Conservatives invited Mr Lit to become 'David Cameron's Conservative candidate'," he said.

Doubts have also been raised about the wisdom of Mr Cameron's flying visit to Rwanda next week, amid fears that the 8,000-mile trip would be seen as a stunt that undermined his green message about flying less.

Last night it was reported that Ed Miliband, the Cabinet Office minister, has started work on Labour's manifesto for the next general election.

Mr Miliband began the task "actively" on Saturday, Downing Street sources told the BBC. The news is likely to increase speculation that the Prime Minister is thinking of calling an early election.